It is known for two discharge lamps to be operated using two load circuits. Here, the load on a bridge which is used as an inverter to operate a discharge lamp is referred to as the load circuit. Each load circuit has a dedicated preheating arrangement for each lamp. The possibility of operating two lamps in one load circuit is also known. Here, the primary coil of a heater transformer is connected in parallel with two lamps connected in series, and the secondary coil of the heater transformer is connected between the two lamps.
The circuitry of the load circuits is comparatively complex since electronic control circuits having relay or transistor switches are required for the defined, sequential starting and subsequent joint operation of the lamps. In order to operate individual lamps, on the other hand, there are comparatively favorable control circuits which use only passive components to control the preheating. An essential constituent of such circuits is a heat-sensitive resistor having a positive temperature coefficient.
FIG. 1 shows a bridge circuit having a load circuit associated with it. For inversion purposes, the bridge is in the form of a half-bridge having two switching elements 1 and 2 and two capacitors 3 and 4. The load circuit 5 in the bridge comprises a coil 6 in series with a lamp 7 which is connected in parallel with both a resonant capacitor 8 and a heat-sensitive resistor 9.
The method of operation of the circuit shown in FIG. 1 is explained below. By driving the switches 1 and 2 in a suitable manner, an a.c. voltage is generated from the d.c. voltage for the load circuit 5 in the central tap of the bridge. For the starting process of the lamp, the frequency of the a.c. voltage is preferably in the region of the resonant frequency of the coil 6 and the capacitor 8. Prior to starting, the resistor 9 having a positive temperature coefficient acts as a PTC thermistor mistuning the series tuned circuit 6, 8 such that the necessary starting voltage across the lamp 7 or the capacitor 8 is not reached. However, current is already flowing through the incandescent filaments 10 and 11 of the lamp 7, with the result that the incandescent filaments 10 and 11 are preheated for the starting process. At the same time, current also flows through the PTC thermistor 9 and heats it in this preheating phase. In the process, the resistance of the PTC thermistor 9 increases, causing the mistuning of the series resonant circuit 6, 8 to be correspondingly reduced, with the result that the starting voltage may be reached across the lamp 7. The PTC thermistor 9 is designed such that even after starting it carries a sufficient amount of current for it to still have a high resistance, with the result that the resonance can be maintained with an appropriate Q-factor.
For the sake of clarity, FIG. 2a shows the load circuit 5 without the coil 6. FIG. 2b shows a variant of the load circuit in FIG. 2a. A series capacitor 12 is connected in series with the PTC thermistor 9. This causes the mistuning of the resonant circuit by the PTC thermistor 9 to be not as pronounced as in the case of the circuit in FIG. 2a. This means that, in this case, the starting voltage is achieved more quickly and, as a result, the lamp starts more quickly.
A further variant of the load circuits shown in FIGS. 2a and 2b is depicted in FIG. 2c. In this case, the series capacitor 12 is the primary governing factor when the PTC thermistor 9 is in the cold state, whereas in the warm state of the PTC thermistor 9, i.e. during operation and starting of the lamp, the primary governing factor is the series circuit of the two capacitors 8 and 9.